Wear resistant boronized surfaces and boronizing methods

ABSTRACT

Disclosed herein are treatments for the manufacture of wear resistant steel surfaces, which are carburized, boronized, quenched and tempered for extreme surface hardness, with a strong, tough supporting base that minimizes fracturing of the brittle boronized case.

United States Patent n91 Scales 1451 Nov. 25, 1975 1 1 WEAR RESISTANTBORONIZED SURFACES AND BORONIZING METHODS [75] Inventor: Stanley R.Scales, Houston, Tex.

[73] Assignee: Hughes Tool Company, Houston,

Tex.

[22] Filed: Aug. 10, 1973 [21] App]. No: 387,566

[52] US. Cl. 308/82; 148/165; 148/315; 175/374 [51] Int. Cl. F16C 19/00[58] Field of Search 308/82, 241; 148/31.5, 148/165; 175/374 [56]References Cited UNlTED STATES PATENTS 3,227,586 1/1966 Spencer 148/165X 3,275,389 9/1966 Neilson et al 308/82 3,795,551 3/1974 Swirnow148/3145 3,811,961 5/1974 Weinstein et a1. 148/315 PrimaryExaminerCharles J. Myhre Assistant Examiner-James D, Liles Attorney,Agent, or FirmRobert A. Felsman [57] ABSTRACT Disclosed herein aretreatments for the manufacture of wear resistant steel surfaces, whichare carburized, boronized, quenched and tempered for extreme surfacehardness, with a strong, tough supporting base that minimizes fracturingof the brittle boronized case.

8 Claims, 2 Drawing Figures U.S. Patent Nov. 25, 1975 3,922,038

WEAR RESISTANT BORONIZED SURFACES AND BORONIZING METHODS BACKGROUND OFTHE INVENTION:

1. Field of the Invention:

This invention relates in general to surface treatments of metals,particularly to those for steels requiring exceptional wear resistanceunder heavy loads such as those imposed upon earth boring drill bitbearings.

2. Description of the Prior Art:

It has long been known that the surfaces of metals such as steel may betreated with a variety of elements or compositions that combine with theconstituents of steel to produce hard wear resistant surfaces. One ofthe known prior art treatments is carburization. Another is boronizing.

The bearing surfaces of rotatable cutters in earth boring drill bits arecommonly carburized, hardened and tempered to increase their wearresistance. Such surfaces sometimes have regions of soft anti-gallingmaterial such as silver or silver alloy, as disclosed in US. Pat. No.3,235,316. The mating surfaces of the opposing bearing shafts ofteninclude deposits of hard metal of cobalt based alloy of the Stelliteseries.

Boronizing of the bearing surfaces of earth boring drill bits has notheretofore been commercially successful, principally due to the brittlenature of the hard surfaces and their shallow depth.

SUMMARY OF THE INVENTION This invention relates to the discovery that awear resistant surface for steel such as a bearing surface in an earthboring drill bit may be constructed advantageously by a process thatincludes carburization, boronizing the resulting carburized surface,hardening in a manner to protect the boronized case from decarburizing,and then tempering. By carburizing and boronizing to the requisitedepths, hardening in a manner to produce a martensitic grain structurein the carburized case, and tempering to thereafter produce temperedmartensite, a surface of extreme hardness results with a strong andtough supporting base. This base minimizes fracturing of the brittleboronized case since it is resistant to deformation and provides asatisfactory support for the boronized case.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a fragmentary perspectiveview, partially in section, showing an earth boring drill bit andtypical bearing which receives the metallurgical treatment describedherein.

FIG. 2 is a cross-sectional view as seen looking along the lines IIII ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The numeral 11 in the drawingdesignates an earth boring drill bit having a plurality of head sectionsor legs 13 joined by suitable means such as welding (not shown) andthreaded at 15 for attachment to a supportive drill string member. Oneof the prior art lubrication and compensator systems 17, such asillustrated in FIG. 1, feeds lubricant through a passageway 19 and intoa bearing region formed between a cantilevered shaft or pin 21 of thehead section and a rotatable cutter 23 having teeth 25 fordisintegrating earth. Suitable seal 2 means 27 prevent the escape oflubricant from the bearing region, as explained in US. Pat. No.3,397,928.

The above mentioned bearings in an earth boring drill often include aprimary friction region 29, an antifriction ball bearing and cutterretainer region 31, and radial and thrust friction bearing regionsdesignated respectively 33 and 35.

FIG. 2 shows in cross-section the cantilevered bearing pin 21 androtatable cutter 23. The cylindrical surface 35 of the bearing pin 21receives the special treatment of the present invention, while theopposing surface of the rotatable cutter 23 receives a prior arttreatment such as that described in US. Pat. No. 3,235,316, preferablythat using silver alloy. This patent teaches that improved frictionbearings for earth boring drill bits result from the use of indentationsfilled with a soft anti-galling material such as silver or silver alloy37 in a surface having a hardness and wear resistance comparable tocarburized and hardened steel.

As previously stated this invention utilizes four primary method orprocess steps: carburizing, boronizing, hardening and tempering, each ofwhich will be described separately by way of example. The initial stepin the method of this invention requires carburization of a steelbearing surface such as the primary friction bearing region 29. One ofthe prior art carburization methods may be used. Gas carburizing is awell known art and is the preferred method. It is described on pp.

93-l 14 of Volume 2 of the 8th Edition of the Metals Handbook, HeatTreating, Cleaning and Finishing (1964, American Society for Metals). Anexample of the gas carburizing of the bearing surfaces of a selectivelycopper plated rock bit head section made of A.I.S.I. 4815 steel is asfollows:

Carburizing temperature: 1700F.

Carburizing time: 9 hours at 1700F.

Carburizing atmosphere: Generated endothermic gas enriched with methaneto have a carbon potential of 1.35% carbon.

A typical analysis of the carburizing gas (atmosphere) is as follows:

20% C0 (C0 about 0.05%)

This produces a carburized case depth of about 0.065

inch with carbon content at the surface about 1.00%.

Pack carburizing is another well known art that may be used. It isdescribed on pp. 114-118 of Volume 2 of the same Metals Handbook. Anexample of pack carburizing of the bearing surfaces of a rock bit headsection made of A.I.S.I. 4815 steel is as follows:

Carburizing compound (packed around the surface to be carburized):Charcoal, (6 to 14 mesh size), energized with about 4% BaCO and about1.5% CaCO Carburizing temperature: 1700F. Carburizing time: Nine hoursat 1700F.

This produces a carburized case depth of about 0.065

inch with carbon content at the surface about 1.00%.

Liquid carburizing is another well known art to the metals industry. Itis described on pp. l33145 of Volume 2 of the same Metals Handbook.While not actually used for this invention, it is anticipated that 9hours at 1700F. in a salt bath containing about 6 16% sodium cyanide and30 55% barium chloride would produce a satisfactory carburized case to adepth of about 0.065 inch.

The second step of the method of this invention is boronizing of thepreviously carburized surfaces. Pack boronizing is the preferredtechnique and is a relatively new art.

An example of boronizing a carburized bearing surface of an A.l.S.l.4815 steel head section is as follows:

Compound: Boronizing powder was packed around the carburized bearingsurface. This powder was 90% finer than 150 mesh, had 4080% 8 C, 240%(graphite), l4% KHCO with remainder up to 20% in impurities.

Boronizing temperature: 1650F. (in a carburizing atmosphere).

Boronizing time: 5 hours at temperature in a furnace with a carbonpotential of 1.00%.

This produced a boronized case depth of about 0.004 inch. Longerboronizing times and/or higherboronizing temperatures can be used fordeeper boronized case depth, but a 0.001 to 0.010 inch deep boronizedcase has less tendency to crack or spall than a deeper case.

Gas boronizing is an alternate technique in the prior art. It isdescribed in U.S. Pat. No. 2,494,267, Surface Hardening of FerrousMetals, Schlesinger and Schaffer, Jan. 10, 1950. The method describedutilizes gaseous diborane (B 11 at about 700C (1292F).

Liquid boronizing is another prior art boronizing method. It isdescribed in two papers: Boronizing of Steel by D. C. Durrill and Dr.Donald D. Allen, Magnetic Propulsion Systems, Inc. and Boriding Steelsfor Wear Resistance by Howard C. Fielder and Richard J. Sieraski,General Electric Co. (Metal Progress, Feb. 1971, pp. 101-107).

Neither gives the liquid salt bath composition, but the latter paperstates that it contains flourides of lithium, sodium, potassium, andboron. Temperatures and times reported vary from l450 1650F. and from 15minutes to 36 hours.

The third and fourth steps in the method of this in-' vention arehardening and tempering of the carburized, boronized and cleaned steelsurfaces.

The hardening and tempering of carburized steel is a well known art. Thehardening, usually quenching in agitated oil, from a temperature of atleast 1390F. can be performed using one of several procedures, such asthe following two for carburized A.l.S.I. 4815 steel and produces amartensitic case:

a. Single quench from a carburizing or reducing atmosphere and atemperature of 1500F.

b. Double quench from a carburizing or reducing atmosphere andtemperatures ofrespectively 1550F. and 1435F.

Quenching from a carburizing or reducing atmosphere preventsdecarburization or oxidation of the boronized case. A suitable coatingsuch as copper plating may be used. A suitable atmosphere is one similarto the previously explained methane enriched endothermic gas exceptslightly higher in CO (about 0.4 to 0.8%) because of the lowertemperatures.

The tempering temperature is usually low, 290 510F., preferably about330F. for 1 hour, to toughen the carburized case without appreciablylowering its strength (hardness) to produce tempered martensite.

The above described procedures are utilized to produce a carburizedfoundation on the steel bearing pin 21 of at least 0.060 inch. Theboronizing procedure produces a boronized case of at least 0.001 inch(average about 0.004 inch) with a surface hardness in a range of 900 to2100 KHN. The hardening and tempering procedure develops a hardness inthecarburized foundation in a range of 50 to 64 Rockwell C (550 to 800KHN). When such a bearing pin is assembled with a rotatable cutter ofthe type shown in the drawing and described in U.S. Pat. No. 3,235,316,overall bearing performance is much improved over that obtainable with acarburized bearing surface Results indicate that such a surface performscomparably with much more expensive procedures such as when utilizing adeposit of hard metal of the cobalt based Stellite series in a groove onthe bearing pin.

While the invention has been described in only a few of its forms itshould be apparent to those skilled in the art that it is not so limitedbut is susceptible to various. changes and modifications withoutdeparting from the spirit thereof.

1 claim:

1. In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises:

a carburized friction bearing surface formed on said steel bearing pin;

a boronized surface formed on said carburized bearing surface;

said carburized surface being hardened and tempered by heat treatment;

whereby the wear resistant boronized surface has a tough and strongsupporting base to sustain large loads imposed by a mating frictionbearing in said rotatable cutter and avoid brittle fracture. 1

2. In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises;

a friction bearing surface formed on said steel bearing pin and having acarburized case of a depth of at least 0.030 inch;

a boronized surface having a depth of at least 0.001

inch formed on said carburized case;

said carburized surface being hardened I and tempered by heat treatment;

whereby the wear resistant boronized surface has a tough and strongsupporting base to sustain large loads imposed by a mating frictionbearing in said rotatable cutter and avoid brittle fracture.

3. In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises:

a carburized friction bearing surface formedon said steel bearing pin;

a boronized surface formed on said carburized bearing surface;

said carburized case being quenched from a temperature to produce amartensitic grain structure;

said carburized case being tempered to produce a tempered martensiticgrain structure;

whereby the wear resistantboronized surface hasa tough and strongsupporting base to sustain large loads imposed by a mating frictionbearing in said rotatable cutter and avoid brittle fracture.

4. In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises:

a friction bearing surface formed on said steel bearing pin and having acarburized case of a depth of at least 0.030 inch;

a boronized surface having a depth of at least 0.001

inch formed on said carburized case;

said carburized case being quenched from a temperature to produce amartensitic grain structure;

said carburized case being tempered to produce a tempered martensiticgrain structure;

whereby the wear resistant boronized surface has a tough and strongsupporting base to sustain large loads imposed by a mating frictionbearing in said rotatable cutter and avoid brittle fracture.

5. In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises:

a friction bearing surface formed on said steel bearing pin and having acarburized case of a depth of at least 0.030 inch;

a boronized surface having a depth of at least 0.001

inch formed on said carburized case;

said carburized case being quenched from a temperature of at least1390F. to produce a martensitic grain structure;

said carburized case being tempered from a temperature within a range of290 through 510F. for about 1 hour to produce a tempered martinsiticgrain structure;

whereby the wear resistant boronized surface has a tough and strongsupporting base to sustain large loads imposed by a mating frictionbearing in said rotatable cutter and avoid brittle fracture.

6. The method of manufacturing earth boring drill bits having at leastone rotatable cutter supported on a steel bearing pin, said methodcomprising the steps of:

forming a friction bearing surface on said bearing pin to produce acarburized case of selected depth;

boronizing said carburized case;

hardening said carburized case;

tempering said carburized case;

whereby the wear resistant boronized surface has a tough and strongsupporting base to sustain large loads and avoid brittle fracture.

7. The method of claim 11 wherein said carburizing is to a depth of atleast 0.030 inch and said boronizing is to a depth of at least 0.001inch.

8. The method of claim 12 wherein said hardening is by quenching from atemperature of at least l390F., and said tempering is from a temperaturewithin a range of 290 through 510F. for about 1 hour.

UNITED STATES PATENT OFFICE @THHCATE OF CORECHN Patent No. 3,922,038Dated November 25, 1975 Inventor(s) Stanley R. Scales It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby Corrected as shown below:

Claim 7, Col. 6, line 16, "11" is changed to --6---; Claim 8, Col. 6,line l9, "12'' is changed to --7--..

Signed and Scaled this Twentieth Day Of July 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofParentsand Trademarks

1. IN AN EARTH BORING DRILL BIT HAVING AT LEAST ONE ROTATABLE CUTTERSUPPORTED ON A STEEL BEARING PIN, THE IMPROVEMENT WHICH COMPRISES ACARBURIZED FRICTION BEARING SURFACE FORMED ON SAID STEEL BEARING PIN; ABORONIZED SURFACE FORMED ON SAID CARBURIZED BEARING SURFACE; SAIDCARBURIZED SURFACE BEING HARDENED AND TEMPERED BY HEAT TREATMENT;WHEREBY THE WEAR RESISTANT BORONIZED SURFACE HAS A TOUGH AND STRONGSUPPORTING BASE TO SUSTAIN LARGE LOADS IMPOSED BY A MATING FRICTIONBEARING IN SAID ROTATABLE CUTTER AND AVOID BRITTLE FRACTURE.
 2. In anearth boring drill bit having at least one rotatable cutter supported ona steel bearing pin, the improvement which comprises: a friction bearingsurface formed on said steel bearing pin and having a carburized case ofa depth of at least 0.030 inch; a boronized surface having a depth of atleast 0.001 inch formed on said carburized case; said carburized surfacebeing hardened and tempered by heat treatment; whereby the wearresistant boronized surface has a tough and strong supporting base tosustain large loads imposed by a mating friction bearing in saidrotatable cutter and avoid brittle fracture.
 3. In an earth boring drillbit having at least one rotatable cutter supported on a steel bearingpin, the improvement which comprises: a carburized friction bearingsurface formed on said steel bearing pin; a boronized surface formed onsaid carburized bearing surface; said carburized case being quenchedfrom a temperature to produce a martensitic grain structure; saidcarburized case being tempered to produce a tempered martensitic grainstructure; whereby the wear resistant boronized surface has a tough andstrong supporting base to sustain large loads imposed by a matingfriction bearing in said rotatable cutter and avoid brittle fracture. 4.In an earth boring drill bit having at least one rotatable cuttersupported on a steel bearing pin, the improvement which comprises: afriction bearing surface formed on said steel bearing pin and having acarburized case of a depth of at least 0.030 inch; a boronized surfacehaving a depth of at least 0.001 inch formed on said carburized case;said carburized case being quenched from a temperature to produce amartensitic grain structure; said carburized case being tempered toproduce a tempered martensitic grain structure; whereby the wearresistant boronized surface has a tough and strong supporting base tosustain large loads imposed by a mating friction bearing in saidrotatable cutter and avoid brittle fracture.
 5. In an earth boring drillbit having at least one rotatable cutter supported on a steel bearingpin, the improvement which comprises: a friction bearing surface formedon said steel bearing pin and having a carburized case of a depth of atleast 0.030 inch; a boronized surface having a depth of at least 0.001inch formed on said carburized case; said carburized case being quenchedfrom a temperature of at least 1390*F. to produce a martensitic grainstructure; said carburized case being tempered from a temperature withina range of 290* through 510*F. for about 1 hour to produce a temperedmartinsitic grain structure; whereby the wear resistant boronizedsurface has a tough and strong supporting base to sustain large loadsimposed by a mating friction bearing in said rotatable cutter and avoidbrittle fracture.
 6. The method of manufacturing earth boring drill bitshaving at least one rotatable cutter supported on a steel bearing pin,said method comprising the steps of: forming a friction bearing surfaceon said bearing pin to produce a carburized case of selected depth;boronizing said carburized case; hardening said carburized case;tempering said carburized case; whereby the weaR resistant boronizedsurface has a tough and strong supporting base to sustain large loadsand avoid brittle fracture.
 7. The method of claim 11 wherein saidcarburizing is to a depth of at least 0.030 inch and said boronizing isto a depth of at least 0.001 inch.
 8. The method of claim 12 whereinsaid hardening is by quenching from a temperature of at least 1390*F.,and said tempering is from a temperature within a range of 290* through510*F. for about 1 hour.